Methods and systems for control of an emission system with more than one SCR region

ABSTRACT

Methods and systems for controlling an emission control system of a vehicle having a first SCR region upstream of a second SCR region are provided herein. One exemplary method includes, in a first mode: adjusting an amount of reductant injected upstream of a first SCR region based on a condition of the first SCR region. The method may also include, in a second mode: adjusting the amount of reductant injected upstream of the first SCR region based on a condition of the second SCR region.

FIELD

The present application relates to methods and systems for emissioncontrol of a vehicle with more than one selective catalytic reduction(SCR) region.

BACKGROUND AND SUMMARY

Selective catalytic reduction (SCR) systems may be used in a vehicle tofacilitate reduction of engine output NOx by a reductant, such as ureaor ammonia. An SCR system involves injecting the reductant upstream ofan SCR catalyst where the reductant, or reductant products, can reactwith NOx to create byproducts such as nitrogen and water. However, thereis a trade-off between an amount of reductant injected and NOxconversion efficiency with SCR systems. Namely, if a large amount ofreductant is injected, a great amount of reductant may be stored on anSCR catalyst, and thus there may be high NOx conversion efficiency.However, there is also a risk of reductant “slipping” to an exhausttailpipe. On the other hand, when a lesser amount of reductant isinjected, NOx conversion efficiency decreases and NOx emissions may beemitted through the exhaust tailpipe.

A NOx reduction system having a first and second catalyst bed in seriesis described in U.S. Patent Application US 2005/0284134A1 (Radhamohan etal). The system includes a first ammonia injector positioned upstream ofthe first catalyst bed and a second ammonia injector downstream of thefirst catalyst bed and upstream of the second catalyst bed. The firstammonia injector is configured to inject an amount of ammonia that isless than a stoichiometric ratio, so as to reduce only a portion of theNOx received at the first catalyst bed. The second injector iscontrolled to inject an amount of reductant so as to remove a portion,or all, of the remaining NOx that flows to the second catalyst bed fromthe first catalyst bed.

In contrast, Applicants have developed systems and methods formanipulating performance of several regions of an SCR system bycontrolling reductant delivered upstream of a first SCR region. Oneexemplary emission control system of a vehicle includes a first SCRregion upstream of a second SCR region. An exemplary method forcontrolling the emission control system includes, in a first mode,adjusting an amount of reductant delivered upstream of the first SCRregion based on a condition of the first SCR region. The method furtherincludes, in a second mode, adjusting the amount of reductant deliveredupstream of the first SCR region based on a condition of the second SCRregion. The first SCR region may be positioned upstream with respect tothe second SCR region, in one example.

By selectively adjusting upstream reductant delivery in this way, it ispossible to adjust the operation and performance of each of a first andsecond SCR regions to provide different modes of operation, depending onoperating conditions. In this way, a high NOx conversion efficiency canbe maintained while controlling a risk of reductant slip over a widerrange of operation conditions, without requiring independent control ofreductant delivery to each SCR region. For example, if the upstreamregion has reduced capacity, it may be desired to provide a higheramount of storage in the downstream region and utilize NOx conversion inthe downstream region to a greater extent. Here, reductant delivery maybe adjusted based on the storage level of the downstream SCR region.However, if the upstream region has increased capacity, it may bedesired to provide a higher amount of storage in the upstream region andutilize NOx conversion in the upstream region to a greater extent. Here,reductant delivery may be adjusted based on the storage level of theupstream SCR region. In this way, overall performance may be improved.

It should be understood that the summary above is provided to introducein simplified form a selection of concepts that are further described inthe detailed description. It is not meant to identify key or essentialfeatures of the claimed subject matter, the scope of which is defineduniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a cylinder of an engine and an emissioncontrol system as described herein.

FIG. 2 is a schematic view of an exemplary SCR system having two SCRregions.

FIG. 3 is an exemplary schematic view of another embodiment of an SCRsystem having two zones.

FIG. 4 is an exemplary schematic view of another embodiment of an SCRsystem having two SCR regions and an oxidation catalyst.

FIG. 5 is a graph showing efficiency of reductant conversion to NOx bytwo different oxidation catalysts.

FIG. 6 is a graph showing a reductant concentration downstream of twodifferent oxidation catalysts.

FIG. 7 is a graph showing NOx conversion efficiency over time by two SCRregions having different washcoat loadings.

FIG. 8 is a flowchart illustrating a high-level example method foroperating an emission control system.

FIG. 9 is a schematic graph of storage capacity of an SCR region varyingwith a temperature of the SCR region.

FIG. 10 is a flowchart illustrating an example method for selecting anemission control strategy.

FIG. 11 is a flowchart illustrating an example method for operating in afirst emission control strategy.

FIG. 12 is a flowchart illustrating an example method for operating in asecond emission control strategy.

FIG. 13 is a flowchart illustrating an example method for operating in athird emission control strategy.

FIG. 14 is a flowchart illustrating an example method for operating in afourth emission control strategy.

FIG. 15 is a schematic graph of reductant concentration at a midbed ofan SCR system and at an exhaust tailpipe.

FIG. 16 is a flowchart illustrating an example method for diagnosing afirst and/or second SCR region of an emission control system.

DETAILED DESCRIPTION OF THE DRAWINGS

Various SCR emission control system configurations are provided directedto balancing high NOx conversion efficiency while reducing reductantslip. An SCR emission control system may include more than one SCRregion in series (such as a first and second SCR region), with reductantdelivery upstream of each of the SCR regions. The reductant delivery maybe a reductant injector, or a reductant generation devices, such as alean NOx trap operating with rich exhaust gas.

Each of the SCR regions may comprise a catalyst bed within an SCRcatalyst, a region within an SCR catalyst, and/or an SCR catalystitself. By including more than one SCR region in series, a risk ofreductant slip from a first SCR region during periods of high reductantinjection may be reduced, as slipped reductant may be caught and storedat the second SCR region instead of being emitted through an exhausttailpipe.

In order to control an SCR region's capacity for converting NOx, SCRregions may be selectively designed to hold a predetermined amount ofwashcoat per unit area or volume. For example, a first SCR region (e.g.,most upstream) may be designed to have a smaller volume and a greaterwashcoat density than a second SCR region. Such a first SCR region maybe a “light-off” catalyst, such that it may be able to quickly achievehigh levels of NOx conversion (e.g., during engine warm-up). In anotherexample, a washcoat loading of a first SCR region may be limited, andthus an amount of reductant storage may be limited. In such a case, anoxidation catalyst may be positioned downstream of a first SCR region tocapture and convert a portion of slipped reductant to NOx, which can besubsequently delivered to the second SCR region downstream of theoxidation catalyst.

Accordingly, several approaches for operating the emission controlsystems having more than one SCR region in series are also providedherein. In some examples, the approaches include monitoring conditionsof the SCR regions, and then operating based on said conditions. Tocontrol NOx conversion efficiency and reductant slip risk, an amount ofinjected reductant can be adjusted, and temperatures of the SCR regionscan also be adjusted.

As one exemplary approach, a storage capacity of each SCR region may bemonitored, and an amount of reductant stored at each SCR region may alsobe monitored. Based on these conditions, the emission control system canselect an appropriate operating mode. For example, if all of the SCRregions have acceptable storage capacity, the system may operate anupstream SCR region at a very high NOx conversion efficiency whilemaintaining an acceptable amount of reductant stored at a downstream SCRregion. In another example, where all of the SCR regions have acceptablestorage capacity, the system may balance an amount of reductant storedat among the SCR regions. In yet another example, if one or more of theSCR regions has unacceptable, or negligible, storage capacity, thesystem may adjust an amount of reductant stored at other SCR region(s).Of course, various other modes of operation may also be used, such asdescribed herein.

It may be appreciated that the present disclosure further includesapproaches for diagnosing an emission control system having several SCR.Such approaches may vary between particular system configurations, andthey may also vary within a particular system configuration, based on anemission control strategy/mode being implemented. For example, if anemission control system is designed with a NOx sensor or reductantsensor downstream of an SCR region that is furthest downstream, theemission control system may perform diagnostics when at least one of theSCR regions is inoperable, such as when storage capacity isunacceptable, or negligible, in order to isolate performance of otherregions. Of course, several sensors may also be provided for performingdiagnostics. Further details of such diagnostics under such conditionsare described herein.

Referring now to FIG. 1, an environment for operation of an emissioncontrol system is shown. Namely, a schematic diagram showing onecylinder of a multi-cylinder engine 10, and an emission control systemas described herein, is illustrated and described in detail.

Engine 10 may be controlled at least partially by a control systemincluding an electronic controller 12 and by input from a vehicleoperator 132 via an input device 130. In this example, input device 130includes an accelerator pedal and a pedal position sensor 134 forgenerating a proportional pedal position signal PP. Combustion chamber30 (e.g., cylinder) of engine 10 may include combustion chamber walls 32with piston 36 positioned therein. Piston 36 may be coupled tocrankshaft 40 so that reciprocating motion of the piston is translatedinto rotational motion of the crankshaft. Crankshaft 40 may be coupledto at least one drive wheel of a vehicle via an intermediatetransmission system. Further, a starter motor may be coupled tocrankshaft 40 via a flywheel to enable a starting operation of engine10.

Combustion chamber 30 may receive intake air from intake manifold 44 viaintake passage 42 and may exhaust combustion gases via exhaust passage48. Intake manifold 44 and exhaust passage 48 can selectivelycommunicate with combustion chamber 30 via respective intake valve 52and exhaust valve 54. In some embodiments, combustion chamber 30 mayinclude two or more intake valves and/or two or more exhaust valves.

In this example, intake valve 52 and exhaust valve 54 may be controlledby cam actuation via respective cam actuation systems 51 and 53. Camactuation systems 51 and 53 may each include one or more cams and mayutilize one or more of cam profile switching (CPS), variable cam timing(VCT), variable fuelling valve timing (VVT) and/or variable fuellingvalve lift (VVL) systems that may be operated by electronic controller12 to vary fuelling valve operation. The position of intake valve 52 andexhaust valve 54 may be determined by position sensors 55 and 57,respectively. In alternative embodiments, intake valve 52 and/or exhaustvalve 54 may be controlled by electric fuelling valve actuation. Forexample, combustion chamber 30 may alternatively include an intake valvecontrolled via electric fuelling valve actuation and an exhaust valvecontrolled via cam actuation including CPS and/or VCT systems.

A fuel injector 66 is shown in a configuration that provides what isknown as direct injection of fuel into the combustion chamber 30. Fuelinjector 66 may inject fuel in proportion to the pulse width of signalFPW received from electronic controller 12 via electronic driver 68.Fuel may be delivered to fuel injector 66 by a fuel system (not shown)including a storage tank, a fuel pump, and a fuel rail.

Intake passage 42 may include a throttle 62 having a throttle plate 64.In this particular example, the position of throttle plate 64 may bevaried by electronic controller 12 via a signal provided to an electricmotor or actuator included with throttle 62, a configuration that iscommonly referred to as electronic throttle control (ETC). In thismanner, throttle 62 may be operated to vary the intake air provided tocombustion chamber 30 among other engine cylinders. The position ofthrottle plate 64 may be provided to electronic controller 12 bythrottle position signal TP. Intake passage 42 may include a mass airflow sensor 120 and a manifold air pressure sensor 122 for providingrespective signals MAF and MAP to electronic controller 12.

As described above, FIG. 1 shows only one cylinder of a multi-cylinderengine, and each cylinder may similarly include its own set ofintake/exhaust valves, fuel injector, etc.

Electronic controller 12 is shown here as a microcomputer includingmicroprocessor unit 102, input/output ports 104, an electronic storagemedium for executable programs and calibration values shown as read-onlymemory 106 in this particular example, random access memory 108, keepalive memory 110, and a data bus. Storage medium read-only memory 106can be programmed with computer readable data representing instructionsexecutable by microprocessor unit 102 for performing the methodsdescribed herein as well as other variants that are anticipated but notspecifically listed.

Electronic controller 12 may receive various signals from sensorscoupled to engine 10, in addition to those signals previously discussed,including measurement of inducted mass air flow (MAF) from mass air flowsensor 120; engine coolant temperature (ECT) from temperature sensor 112coupled to cooling sleeve 114; a profile ignition pickup signal (PIP)from Hall effect sensor 118 (or other type) coupled to crankshaft 40;throttle position (TP) from a throttle position sensor; and absolutemanifold pressure signal, MAP, from sensor 122. Manifold pressure signalMAP from a manifold pressure sensor may be used to provide an indicationof vacuum, or pressure, in the intake manifold. Engine speed signal,RPM, may be generated by electronic controller 12 from signal PIP. Inone example, the engine position sensor (Hall effect sensor 118) mayproduce a predetermined number of equally spaced pulses every revolutionof the crankshaft from which engine speed (RPM) can be determined.

Turning now to an emission control system downstream of the combustionchamber 30, an exhaust gas sensor 126 is shown coupled to exhaustpassage 48 upstream of an emission control device 70. Emission controldevice 70 may be an oxidation catalyst, NOx trap, diesel particulatefilter (DPF), various other emission control devices, or combinationsthereof. Emission control device 70 can include multiple catalystbricks, in one example. In another example, multiple emission controldevices, each with multiple bricks, can be used. Emission control device70 is shown arranged along exhaust passage 48 downstream of exhaust gassensor 126. Exhaust gas sensor 126 may be any suitable sensor forproviding an indication of exhaust gas air/fuel ratio such as a linearoxygen sensor or UEGO (universal or wide-range exhaust gas oxygen), atwo-state oxygen sensor or EGO, a HEGO (heated EGO), a NOx, HC, or COsensor. In some embodiments, during operation of engine 10, emissioncontrol device 70 may be periodically reset by operating at least onecylinder of the engine within a particular air/fuel ratio ortemperature.

The emission control system further includes a selective catalyticreduction (SCR) system 76 having two or more SCR regions, such as firstSCR region 78 and second SCR region 80. A reductant injector 74 mayinject reductant, such as urea or ammonia, upstream of the first SCRregion 78, according to signals received from the electronic controller12. Reductant injected by the reductant injector 74 may be sourced froma reductant storage unit (not shown).

A sensor 90 may be positioned downstream of the second SCR region 80 (ora last SCR region, where there are more than two SCR regions), and maybe configured to measure NOx, ammonia, and/or other exhaust components,and to communicate the measurements to the electronic controller 12.Thereafter, exhaust gas may flow to remaining downstream componentsand/or an atmosphere via exhaust tailpipe 202.

The electronic controller 12 may receive signals from sensor 90, as wellas send and receive signals from the reductant injector 74 according toa reductant injection strategy. Such a reductant injection strategy maybe updated based on signals received from sensor 90, as will bedescribed later. Also, ammonia may be detected in the exhaust tailpipeusing an excitation method.

One approach for monitoring and controlling the emission control systemincludes employing one or more models of the emission control system orcomponents thereof, at the electronic controller 12. As one example, apredictive or feed-forward ammonia storage model of a first SCR region78 can be used with mapping tables to establish inputs and storagelevels for the SCR regions. In this way, reductant slip from the firstSCR region and/or second SCR region can be effectively controlled. Theusage of reductant storage models also allows for storage of catalystconditions at the electronic controller 12 upon start-up of the vehicleto thereby allow for improved cold-start or hot-start conditions.

Another reductant storage model for the SCR regions may model each SCRregion as having multiple zones, such that control of the emissioncontrol system can be based on local conditions of radial and/or axialareas of the SCR regions. With predictive capabilities, the limits ofreductant injection control can be improved. Through such multi-zonemodeling (or through direct measurement, in some conditions), an SCRregion may be treated as several linked zones with each having specificcharacteristics mapped based on local operating conditions. Also, anoxidation model for an oxidation catalyst in the emission control systemmay improve a predictive nature of storage models of the SCR regions.

Models of the emission control system or components thereof may beupdated based on input received from one or more NOx sensors, reductantsensors, UEGO sensors, and/or temperature sensors positioned upstreamand/or downstream of the first and/or second SCR regions. Feedback fromsensors may also provide validation control for models, and/or may beused to make accurate adjustments to an emission control strategy inreal-time. Further, feedback from sensors may be used to adjust modelinginputs to compensate for any deterioration of SCR region performance,and thus to maintain usefulness of the models over a lifetime of theemission control system.

As mentioned above, multiple configurations of an emission controlsystem having two or more SCR regions in series may be used, and each ofthe configurations may offer specific features. However, by mechanicallyarranging the SCR regions as described herein, some passive control ofreductant slip reduction and NOx conversion efficiency improvements maybe achieved. As one specific example, reductant can be injected upstreamof the first SCR region in amounts conducive to high NOx conversionefficiency at a first upstream SCR region, while excess reductant can bepassively caught and stored at a second (or third, fourth, etc.)downstream SCR region.

As a first example configuration, the emission control system of FIG. 1shows the SCR system 76 including a first SCR region 78 as a firstcatalyst bed and a second SCR region 80 as a second catalyst bed. Thefirst SCR region 78 is substantially smaller in volume than the secondSCR region to allow for rapid light-off. In one example, a total SCRvolume (e.g., volume of first SCR region and second SCR region) may be200% of the engine swept volume, and the first SCR region 78 may have avolume that is any fraction of the total SCR volume that providessufficient storage for cold-start emission levels. The size of thesecond SCR region 80 may be designed to provide sufficient space forstorage of reductant slipped from the first SCR region 78. As the sizeof the second SCR region 80 is increased, a greater overflow reservoirfor ammonia slip from the first SCR region 78 can be provided, and arisk of ammonia slip can be further reduced.

FIG. 2 shows an alternate configuration for the SCR system 76. Here, afirst SCR region 78 is positioned downstream of a reductant injector(not shown) and is substantially separate from the second SCR region 80,which is positioned downstream of the first SCR region 78. That is, thefirst SCR region 78 may be considered a first SCR catalyst and a secondSCR region 80 may be considered a second SCR catalyst.

The second SCR region 80 may be positioned substantially distant fromthe first SCR region 78 such that effects of exhaust gas temperature onthe second SCR region 80 are reduced and/or delayed compared to theeffects of exhaust gas temperature on the first SCR region 78. Thus,during engine operation, if a first SCR region's temperature increasesand the reductant storage capacity of the first SCR region accordinglydecreases, at least the temperature of the second SCR region 80 can bekept lower than the first SCR region 78, so that a predetermined amountof reductant storage at the second SCR region 80 can be maintained.Thus, high NOx conversion efficiency of unconverted NOx passed throughfrom the first SCR region 78 can be achieved at the second SCR region80. In another example, the second SCR region may be coupled to a coolerto maintain a lower temperature than the first SCR region.

FIG. 3 shows another configuration of an SCR system 76. Here, the firstSCR region 78 is shown as a first zone of an SCR catalyst and the secondSCR region 80 is a second zone of the SCR catalyst. In this example, thefirst zone and the second zone are positioned adjacent to one anothersuch that together, they form an integral SCR catalyst. Thisconfiguration is advantageous for reducing space and cost.

Further still, FIG. 4 shows another configuration. The first SCR region78 is positioned downstream of a reductant injector (not shown), and anSCR filter 82 is positioned downstream of the first SCR region 78. Thefirst SCR region 78 may be integrated with the SCR filter 82. The secondSCR region 80 is positioned downstream of the first SCR region 78 andthe SCR filter 82, and an oxidation catalyst 84 is positioned betweenthe SCR filter 82 and the second SCR region 80. With this configuration,one or more sensors (e.g., NOx, NH3, UEGO, etc.) may be located upstreamand/or downstream of the oxidation catalyst 84 so that reductant (e.g.,ammonia) slip may be detected and so that conversion levels of theoxidation catalyst 84 can be detected. A first reductant sensor 86 and asecond reductant sensor 88 may be positioned as shown.

The oxidation catalyst 84 is provided in order to receive slippedreductant from the first SCR region 78 and oxidize at least a portion ofthe slipped reductant. In this way, reductant (e.g., ammonia) can beconverted to NOx for subsequent delivery to the second SCR region 80, toavoid exceeding a reductant storage capacity at the second SCR region80. However, under some conditions, it may be desirable to oxidize onlya portion of the slipped reductant at the oxidation catalyst 84, andpurposefully pass a majority of the reductant slipped from the first SCRregion 78 to the second SCR region 80. To achieve this, the oxidationcatalyst 84 may be altered from a traditional oxidation or ammonia slipcontrol function, to one that slips a majority of the ammonia from thefirst SCR region 78 or SCR filter 82 to the second SCR region 80.

Accordingly, the oxidation catalyst 84 may include materials which causethe reductant-to-NOx conversion to be an inefficient process (e.g., 40%reductant-to-NOx conversion). For example, the oxidation catalyst 84 maybe thin, and may thus have relatively lower efficiency due to limitedresidence time for emissions to interact with a catalytic surface.Alternatively, or additionally, there may be holes drilled in theoxidation catalyst brick to create gas bypass regions, thereby reducingopportunity for exhaust bulk exchange with the catalyst washcoat. Forexample, the oxidation catalyst 84 may include a perforated material ormay be designed so as to form and/or include a plurality of holes. Inanother example, the oxidation catalyst 84 may be formed with a low celldensity, such as 50-200 cpsi, thereby limiting bulk transfer ofreductant (e.g., ammonia) to the oxidation catalyst 84 for oxidation.Further still, the oxidation catalyst 84 may have an amount of platinumgroup metals below a predetermined threshold tuned for lower oxidationperformance. In still another example, a selective coating process maybe used where, rather than cutting sections from a brick, regions of abase substrate would not be treated or coated with washcoat to createthe non-reactive void regions. This can be done by applying slurrycharges to selective portions of the substrate and not fully coating.Various combinations of the oxidation catalyst designs and materials maybe implemented.

As mentioned above, the oxidation catalyst may be designed so that amajority of the reductant slipped from the first SCR region 78 is passedto the second SCR region 80. FIG. 5 illustrates ammonia (NH₃) conversionto NOx by two different oxidation catalysts, as a function oftemperature. For this exemplary graph, temperature was increased at arate of 10 degrees Celsius per minute, with a 300 ppm ammonia feedgas.The solid curve illustrates that, once a full oxidation catalyst (e.g.,standard diesel oxidation catalyst) has achieved light-off (e.g.,150-200 degrees Celsius), it almost completely converts ammonia to NOx.In this example, the amount of ammonia conversion is expressed as apercentage of the amount of ammonia detected by, for example, reductantsensor 86 of FIG. 4 to an amount of reductant detected by the reductantsensor 88 of FIG. 4, in one example. In contrast, the dashed curveillustrates that once an oxidation catalyst with 75% of its surfaceavailable for oxidation has achieved light-off, it convertsapproximately 50% of the ammonia that it receives. Thus, if it isdesired to pass a majority of the reductant that is received at theoxidation catalyst 84 to the second SCR region 80, the oxidationcatalyst with 75% of its surface available for oxidation may be includedin the SCR system 76. An amount of an available catalytic surface may beselected for based on a desired amount of ammonia conversion at theoxidation catalyst, and any conversion percentage is possible.

Further, FIG. 6 illustrates an amount of ammonia (NH₃) slip downstreamof the diesel oxidation catalyst 84 under the same conditions as thoseof FIG. 6. A full oxidation catalyst slips only a small amount ofammonia once it has reached a steady operating state (e.g., 300 degreesCelsius). However, an oxidation catalyst with 75% of its catalyticsurface available for oxidation may slip approximately half of theammonia that the oxidation catalyst is fed, once it has reached a steadyoperating state, which is consistent with the amount of ammoniaconversion illustrated in FIG. 5.

It may be appreciated that an oxidation catalyst may include one or moreof an ammonia oxidation catalyst, a lean NOx trap, a diesel oxidationcatalyst and/or a NOx adsorption catalyst. As will be discussed later,oxidation catalyst performance feedback to an electronic controllerand/or to a reductant injector may be used to control reductantinjection.

Turning now to the issue of NOx conversion at each of the SCR regions ofthe SCR system configurations presented, in one example a first SCRregion is operated to convert a relatively higher portion of engineoutput NOx, as compared to a second (or third) SCR region. One way toachieve this is to have a highly loaded washcoat at the first SCR regionso that an amount of NOx converted at the first SCR region can beincreased. In order to achieve a high loaded washcoat and still be ableto achieve rapid light-off (e.g., before light-off of the second SCRregion 80), a first SCR region may have a smaller volume than a secondSCR region (as shown in FIG. 1).

Accordingly, a first SCR region may have a first surface area-volumeratio that is greater than a second surface area-volume ratio of asecond SCR region. In other words, a first washcoat density of the SCRregion may be greater than a second washcoat density of the second SCRregion. In one example, a first SCR region may have a coating level thatis in excess of the standard loading level by at least 10% and thecoating level may be as high as the injection limit allows for rapidsaturation of reductant (e.g., ammonia, urea). In this way, even if afirst SCR region is small in volume, the first SCR region can be quicklysaturated with reductant injected during an engine cold start, andlight-off of the first SCR region can be rapidly achieved due to lowthermal inertia.

As shown in FIG. 7, by increasing a loading of the washcoat of a firstSCR region by ˜30%, the time to achieve light-off operation of asaturated SCR region can be reduced. That is, the high loaded washcoat(solid curve, 1.3×) causes NOx removal to occur significantly soonerthan NOx removal by the standard washcoat load (dashed curve, 1.0×).

To achieve a high washcoat density, several techniques may be employed.For example, a first SCR region may include high cell density materialsfor increasing the available surface area to apply washcoat. This mayallow for more bulk-surface interface for increased exchange onperformance. As another example, a first SCR region may have highporosity materials such that additional washcoat can be supported withinthe wall structure of the first SCR region. In this way, washcoat can bein the wall and on the surface to provide for maximum loading. As yetanother example for increasing washcoat loading, a first SCR region mayinclude thin-wall materials. An advantage of applying the SCR washcoatto a thin-wall material is to reduce the thermal inertia of the SCRcomponent such that the washcoat material heats up and retains the heatsooner.

Further, with respect to an extruded SCR having no substrate can be anadditional option to create a high washcoat density SCR. It may beappreciated that various combinations, or none, of the designs andmaterials discussed for increasing washcoat loading may be implemented.A balance of these features can be tuned to maintain a desired full-lifeperformance of the emission control system components.

In contrast, sometimes washcoat loading, or washcoat density, of an SCRregion may be purposefully limited to avoid excessive back-pressure. Insuch a case, an emission control system with two or more SCR regions inseries may still have a finite capacity for ammonia storage and thuscarry a risk of slipping reductant from a second SCR region to theexhaust tailpipe. In this example, where pressure relief in the systemis desired, the usage of an oxidation catalyst (see FIG. 4) may beadvantageous because, as described above, a first SCR region can besaturated to achieve quick light-off and at least some of the slippedreductant from the first SCR region can be converted to NOx at theoxidation catalyst, thereby reducing a risk of reductant being slippedfrom the second SCR region.

Several emission control system configurations for passively reducingammonia slip while improving NOx conversion efficiency have beenpresented. Each of these configurations may correspond with particularemission control strategies. That is, to further address reductant slip,several methods for actively preventing ammonia slip while improving NOxconversion efficiency are provided, and still various others may also beused.

Specifically, FIG. 8 illustrates an overview method 500 for selectingone of a plurality of emission control strategies based on SCR regionconditions, such as storage capacity. The plurality of emission controlstrategies are presented at FIGS. 10-14. Specifically, a first emissioncontrol strategy may be selected and carried out if a first SCR regionand a second SCR region are operating under steady state, warmed-up,conditions (e.g., both SCR regions have a desired storage capacity). Asecond emission control strategy may be selected and carried out if thefirst SCR region does not have the desired storage capacity (e.g., toolow) but the second SCR region has a desired storage capacity, such aswhen a first SCR region is over-heated. Further, a third emissioncontrol strategy may be selected and carried out if the first SCR regionhas a desired storage capacity but the second SCR region does not have adesired storage capacity (e.g., too low), such as during engine warm-up.Further still, a fourth emission control strategy may be selected ifboth SCR regions do not have desired storage capacities, such as if oneor more SCR regions are degrading, or degraded.

Turning now to FIG. 8, at 502, operating conditions are read. Operatingconditions may include one or more of engine operating conditions,emission control system conditions, etc. At 504, a storage capacitythreshold (e.g., a minimum desired storage capacity) is set for a firstSCR region and for a second SCR region.

A storage capacity threshold may be determined based on an engineoperating condition, such as whether the engine is starting up, idling,accelerating/decelerating, shutting down, etc. Different engineoperating conditions may result in different engine output NOx levels,and so a storage capacity threshold for an SCR region may be set toaccommodate a current engine output NOx level. For example, a greaterstorage capacity threshold may be set during engine warm-up, when thereis high engine output NOx, compared to during engine idling.Furthermore, a storage capacity threshold may be determined based on adesired emission control strategy for the emission control system, andthe desired emission control strategy may itself be based on a conditionof the first and/or second SCR regions, as will be discussed. It may beappreciated that a storage capacity threshold may ultimately be limitedby washcoat loading and/or washcoat density of an SCR region.

Turning to FIG. 9, the graph shows how a storage capacity of an SCRregion may vary with temperature. Thus, a storage capacity (SC)threshold for an SCR region may be set, at 504 of method 500, by settinga desired temperature range of the SCR region, such as between T_(1L)(e.g., a lower temperature threshold) and T_(1H) (e.g., an uppertemperature threshold). The storage capacity threshold for SCR regionsmay be determined and set at the electronic controller based onpre-stored maps, such as the exemplary graph of FIG. 9, or based on oneor more algorithms or models. Although FIG. 9 illustrates an exemplarycurve of storage capacity by temperature for a first SCR region at FIG.9, it may be appreciated that an exemplary relationship between storagecapacity and temperature for a second SCR region may be similar ordifferent from that shown in FIG. 9.

At 506, the method 500 may include determining emission controlconditions. As one example, emission control conditions may include anactual storage capacity of each of the first and second SCR regions.Accordingly, the method 500 includes, at 508, selecting and/or settingan emission control strategy based on the emission control conditionsdetermined at 506.

Turning now to FIG. 10, steps 506 and 508 of method 500 are described inmore detail. Specifically, FIG. 10 shows an exemplary method 700 fordetermining storage capacity conditions of a first and/or second SCRregion and selecting an emission control strategy. In general, storagecapacity conditions may be determined by a temperature of an SCR region,which may be associated with a particular storage capacity value orrange of values, as was described with respect to FIG. 9.

At 702, the method 700 includes determining if the storage capacity ofthe first SCR region is greater than the first storage capacitythreshold. Specifically, this may include determining if a temperatureof the first SCR region is above a lower threshold temperature T_(1L)and/or if the temperature of the first SCR region is below an upperthreshold temperature T_(1H).

If the answer is yes at 702, such that there is a desired amount ofstorage capacity in the first SCR region, the method 700 proceeds to704, where it is determined if the storage capacity of the second SCRregion is greater than a second storage capacity threshold. If theanswer is yes at 704, such that there is a desired amount of storagecapacity in the second SCR region, the method 700 proceeds to 706 wherethe first emission control strategy is selected. The first emissioncontrol strategy may be selected when both first and second SCR regionsare warmed up and are capable of converting engine output NOx tonitrogen, water, and carbon dioxide in the presence of urea, forexample. This may be considered a steady state of operation. As will bediscussed below with respect to FIG. 11, the first emission controlstrategy may include controlling the first SCR region and/or the secondSCR region to maintain a predetermined amount of stored reductant ateach of the SCR regions (e.g., each SCR region may have a differentpredetermined amount, one higher than the other, or they be the same).

If the answer to 702 is no, such that the storage capacity of the firstSCR region is less a desired amount, the method 700 proceeds to 708where it is determined if the storage capacity of the second SCR regionis greater than the second storage capacity threshold. If the answer isyes at 708, such that there is a desired amount of storage capacity atthe second SCR region, the method 700 proceeds to 710 where a secondemission control strategy is selected. Conditions for the selection ofthe second emission control strategy at 710 may transpire, for example,when an engine has been running for an amount of time so as to cause thefirst SCR region to be very hot (above an upper temperature threshold,for example), but the second SCR region is still sufficiently cool toconvert NOx at an acceptable rate. As will be discussed in detail withrespect to FIG. 12, the second emission control strategy may includecontrolling the second SCR region to maintain a predetermined amount ofstored reductant at the second SCR region and/or to bring the storagecapacity of the first SCR region above the first storage capacitythreshold.

If the answer is no at 704, such that the storage capacity of the firstSCR region is greater than the first storage capacity threshold (e.g., adesired storage capacity), but the second SCR region is not greater thanthe second storage capacity threshold (e.g., not a desired storagecapacity), the method 700 proceeds to 712, where a third emissioncontrol strategy is selected. Conditions for selection of the thirdemission control strategy at 712 may transpire during an engine warm-up,for example, when the first SCR region has achieved light-off but thesecond SCR region has not yet achieved light-off. The third emissioncontrol strategy, discussed in detail with respect to FIG. 13, mayinclude controlling the first SCR region to maintain a predeterminedamount of stored reductant therein and/or increasing the storagecapacity of the second SCR region to be above the second storagecapacity threshold.

If the answer is no at 708, such that both the first and second SCRregions have a storage capacity that is less than a respective storagecapacity threshold (e.g., not a desired storage capacity), the method700 proceeds to 714, where a fourth emission control strategy isselected. The fourth emission control strategy may be selected at 714,for example, if both of the SCR regions are above a high temperaturethreshold, and thus are unable to convert NOx at an acceptable rate. Thefourth emission control strategy may include efforts to increase one ormore of the SCR regions' storage capacity to above a storage capacitythreshold, as will be discussed in more detail with respect to FIG. 14.

Although components of method 700 are shown in one particular order, itmay be appreciated that some or all of the actions may be included inthe routine, and the actions may be carried out in any order.

The emission control strategies that will be described with respect toFIGS. 11-14 offer increased flexibility for an SCR system. Under someconditions, a reductant can be over-injected (with respect to astoichiometric relation for near-complete NOx conversion) in order tosaturate a first SCR region with reductant, thereby ensuringnear-complete or complete NOx conversion efficiency. Under otherconditions, reductant can be under-injected to reduce a risk of ammoniaslip and/or to substantially deplete one or more SCR regions of storedreductant. At still other times, reductant can be injected at a levelthat is stoichiometric with respect to an amount needed to substantiallycompletely convert NOx.

Referring now to FIG. 11, the first emission control strategy isillustrated. Method 800 may be carried out when the engine is operatingin a steady state, for example. Specifically, as discussed with respectto FIG. 10, the method 800 may be carried out when the storage capacityof the first SCR region is above a first storage capacity threshold andthe storage capacity of the second SCR region is above a second storagecapacity threshold.

At 802, the method 800 includes determining if an amount of reductantstored at the first SCR region is greater than an upper threshold amountA1, which is schematically indicated on FIG. 9 by a dashed line. Such anupper threshold amount A1 may be set such that a “safety margin” existsabove A1. By setting A1 to be lower than a maximum storage capacity, asituation where no more reductant can be stored and a risk of reductantslip is very high, can be avoided. In some examples, the amount ofstored reductant may be calculated or predicted by a reductant storagemodel as described above. In this way, reductant slip from the first SCRregion can be minimized. If the answer is no at 802, indicating thatreductant stored at the first SCR region is below the upper thresholdamount A1, the method 800 proceeds to 804.

Similarly, at 804, the method 800 includes determining if the amount ofreductant stored at the second SCR region is greater than an upperthreshold A2, which may be selected or determined in a manner similar toupper threshold A1. That is, upper threshold A2 may also be selectedsuch that a “safety margin” exists above A2. If the answer is no at 804,the method 800 includes adjusting a reductant injection, or adjusting anamount of reductant injected upstream of a first SCR region based on acondition of the first SCR region, such as an amount of stored reductantat the first SCR region. In other examples, a condition of the first SCRregion may include a storage capacity, a degradation, a volume, adensity, etc. of the first SCR region.

The adjusting of 806 may be carried out to achieve a desired NOxconversion efficiency at the first SCR region. For example, during anengine start, the adjusting of 806 may include adjusting to inject anamount of reductant that is greater than a stoichiometricreductant-to-NOx ratio. That is, the first SCR region may be saturatedwith reductant such that very high NOx conversion efficiency (e.g., X %)can be achieved at the first SCR region during periods of high engineoutput NOx. In such a case of high reductant injection, at least somereductant (e.g. Y %) may slip from the first SCR region to the secondSCR region, where it can be caught and stored for future use.

In some cases, the adjusting of 806 may include adjusting the injection,at 808, based on a condition of the second SCR region, such as an amountof reductant stored at the second SCR region. This may assist inmaintaining an amount of reductant stored at the second SCR region at adesired level (e.g., below a second threshold amount). Further still, ingeneral, when an oxidation catalyst is included in an emission controlsystem, an amount of reductant injection may be further adjusted basedon the performance of the oxidation catalyst.

It may be appreciated that the adjusting of 806 may be carried out, insome instances, when a storage capacity of the first SCR region isgreater than a storage capacity of the second SCR region.

If the answer is yes at 804, indicating that an amount of storedreductant at the first SCR region is below an upper threshold amount A1,but that the amount of stored reductant at the second SCR region isequal to or greater than an upper threshold amount A2, actions may becarried out to reduce the amount of reductant stored at the second SCRregion by increasing NOx flow to the second SCR region.

Specifically, at 810, the method 800 may include reducing a reductantinjection by a reductant injector to thereby reduce the amount of storedreductant at the first SCR region to a predetermined low level. Themethod 800 may further include increasing a temperature of the first SCRregion to reduce a capability of the first SCR region to convert NOx at812. In this way, NOx flow to the second SCR region can be increased.The temperature may be increased at 812 in accordance with an amount ofstored reductant at the second SCR region. In this way, the amount ofNOx slipped to the second SCR region can be increased for consumption byreductant stored at the second SCR region.

In some cases, the method 800 may actually include, at 812, disablingthe first SCR region (e.g., by increasing temperature of the first SCRregion to a sufficiently high level) so that substantially all of theNOx (e.g., Z %) produced by the engine is passed to the second SCRregion. The disabling at 812 may also be carried out by otherwisedecreasing the amount of stored reductant at the first SCR region. Byreducing the amount of reductant injected (such as at 810) before theincreasing of the temperature of the first SCR region (such as at 812),a risk of reductant slip from the first SCR region to the second SCRregion upon increasing of the temperature of the first SCR region at 812can be reduced or eliminated.

At 814, the method 800 may include increasing NOx flow to the second SCRregion by making the air-fuel ratio of the combustion chamber contentsmore lean. At 816, the method 800 may include further reducing an amountof reductant injected by a reductant injector. In this way, less NOx canbe converted at the first SCR region, and this may effectively result inmore NOx being passed to the second SCR region. By doing this, reductantstored at the second SCR region is used.

In the case of the emission control system including an oxidationcatalyst positioned between the first SCR region and the second SCRregion, the method 800 may include oxidizing some reductant at theoxidation catalyst. Thus, NOx flow to the second SCR region can also beincreased by oxidation of reductant at the oxidation catalyst. In someexamples, the method 800 may include injecting an amount of reductantthat is greater than a stoichiometric reductant-to-NOx ratio, toincrease an amount of NOx generated by the oxidation catalyst.

If the answer to 802 is yes, and the amount of reductant stored at thefirst SCR region is equal to or greater than the upper threshold amountA1, actions to reduce the amount of reductant stored at the first SCRregion may be carried out. Namely, at 820, an amount of injectedreductant may be reduced. Furthermore, at 822, the method 800 mayinclude increasing engine output NOx, and thereby increasing NOx flow tothe first SCR region by, for example, operating the engine at a leanerair-fuel ratio. In another example, when the amount of reductant storedat the first SCR region is equal to or greater than the upper thresholdamount A1, the amount of reductant stored at the second SCR region maybe determined, and reductant injection, NOx flow, and/or temperature ofthe first and/or second SCR regions may be further adjusted based on theamount of reductant stored at the second SCR region.

In some emission control configurations, it may be desirable to includea reductant slip catalyst downstream of the second SCR region to allowthe emission control system to operate with an amount of storedreductant near a maximum storage capacity of each respective SCR region.That is, the amount of reductant injected by the fuel injector would besufficiently high to keep the washcoat of both of the first and secondSCR regions saturated. In this case, if the emission control systemincludes an ammonia slip catalyst, and the answer is yes at 802, steps820 and 822 may be skipped, and the system may be monitored for ammoniaslip in the tailpipe. If an amount of ammonia slip in the tailpipe isabove a predetermined threshold, an exhaust temperature, engine outputNOx, reductant injection, and/or reductant storage capacity of the firstand/or second SCR region can be adjusted to effectively purge anydesired region of the emission control system.

Turning to FIG. 12, the second emission control strategy is illustratedas an exemplary method 900. As discussed above, the second emissioncontrol strategy may be carried out when the storage capacity of thefirst SCR region is less than the first storage capacity threshold,(e.g., not a desired storage capacity), and the storage capacity of thesecond SCR region is above a second storage capacity threshold (e.g., adesired storage capacity), which may occur when the first SCR region isover-heated. In some cases, method 900 may be carried out when thestorage capacity of the first SCR region is smaller than the storagecapacity of the second SCR region, where the first and second SCRregions are positioned in separate emission control devices.

At 902, the method 900 includes determining if an amount of reductantstored at the second SCR region is less than the upper threshold amountA2. If so, the method proceeds to 904, including adjusting an amount ofreductant injected upstream of the first SCR region based on a conditionof the second SCR region, such as an amount of reductant stored at thesecond SCR region. In other examples, a condition of the second SCRregion may include a storage capacity, a degradation, a volume, adensity, etc. of the second SCR region.

However, if the amount of reductant stored at the second SCR region isequal to or greater than the upper threshold amount A2, the method 900proceeds to 906, where engine output NOx is increased in an attempt toreduce the amount of stored reductant at the second SCR region. This maybe achieved by, for example, causing one or more combustion chambers tooperate more lean. If the emission control system includes an oxidationcatalyst, the method 900 may include oxidizing some reductant at theoxidation catalyst at 908 to further increase NOx flow to the second SCRregion. Further, the reductant injection may be increased so as toincrease an amount of NOx produced by the oxidation catalyst. The method900 may alternately include reducing the reductant injection at 910, ifan oxidation catalyst is not a part of the emission control system.

The second emission control strategy presented as method 900 may alsoinclude efforts to increase the storage capacity of the first SCR regionto above the first storage capacity threshold. This may be done forexample, by controlling a temperature of the first SCR region, asstorage capacity is largely affected by temperature. Therefore, it isdetermined at 912 if a temperature of the first SCR region is greaterthan a low temperature threshold T_(1L). If the answer is no at 912,this may indicate the first SCR region is too cold, accounting for thelow storage capacity. Thus, the method 900 proceeds to 914 where thefirst SCR region is warmed up by increasing the exhaust temperature inone example. If the answer is yes at 912, this may indicate the firstSCR region is too hot, accounting for the low storage capacity.Accordingly, the method 900 proceeds to 916 where the exhausttemperature may be reduced in an attempt to cool the first SCR region.Controls may be in place so that that the exhaust temperature is notincreased too much at 914 nor decreased too much at 916 so as tosubstantially and/or negatively affect operation of other emissioncontrol devices (e.g., oxidation catalyst, second SCR region, etc.).Other techniques for increasing or decreasing the temperature of thefirst SCR region may be employed, such as use of one or more heatexchangers.

Referring now to FIG. 13, the third emission control strategy isillustrated as an exemplary method 1000. As discussed above, the thirdemission control strategy may be carried out when the storage capacityof the first SCR region is above a first storage capacity threshold(e.g., a desired storage capacity), and the storage capacity of thesecond SCR region is not above a second storage capacity threshold(e.g., not a desired storage capacity), such as when the engine iswarming up so that the first SCR region has achieved light-off but thesecond SCR region has not yet achieved light-off. In some cases, method1000 may be carried out when the storage capacity of the first SCRregion is greater than the storage capacity of the second SCR region.

At 1002, the method 1000 includes determining if the amount of reductantstored at the first SCR region is less than the upper threshold amountA1, which is schematically indicated on FIG. 9. If the answer is yes at1002, the method 1000 proceeds to 1004. At 1004, the method 1000includes adjusting an amount of reductant injected upstream of a firstSCR region based on a condition of the first SCR region, such as anamount of stored reductant at the first SCR region. The adjusting of1004 may be carried out to achieve a desired NOx conversion efficiencyat the first SCR region. For example, during an engine start, theadjusting of 1004 may include adjusting to inject an amount of reductantthat is a stoichiometric reductant-to-NOx ratio. Thus, near-complete NOxconversion efficiency (e.g., greater than 90%) can be achieved at thefirst SCR region during periods of high engine output NOx.

If the answer is no at 1002, indicating that an amount of storedreductant at the first SCR region is equal to or greater than an upperthreshold amount A1, actions may be carried out to reduce the amount ofreductant stored at the first SCR region by increasing NOx flow to thefirst SCR region. As discussed above, this may be achieved by operatingthe one or more of the combustion chambers of the engine at a leanerair-fuel ratio.

At 1008, the method 1000 may also include reducing a reductant injectionby a reductant injector to thereby reduce the amount of stored reductantat the first SCR region to below the upper threshold amount A1.

The third emission control strategy presented as method 1000 may alsoinclude efforts to bring the second SCR region's storage capacity abovethe second storage capacity threshold. This may be done for example, bycontrolling a temperature of the second SCR region, as storage capacityof the second SCR region may be adjusted by controlling its temperature.Therefore, it is determined at 1010 if a temperature of the second SCRregion is greater than a low temperature threshold for the second SCRregion T_(2L). If the answer is no at 1010, this may indicate the secondSCR region is too cold, accounting for the low storage capacity. Thus,the method 1000 proceeds to 1012 where the second SCR region is warmedup, for example, by increasing the exhaust temperature.

If the answer is yes at 1010, it may be determined that the second SCRregion may be too hot, since the storage capacity is low and the secondSCR region is not too cold. Thus, the method proceeds to 1014 where theexhaust temperature may be reduced in an attempt to cool the second SCRregion and thus increase the storage capacity. Controls may be in placesuch that the exhaust temperature is not increased too much at 1012 nordecreased too much at 1014 so as to substantially and/or negativelyaffect operation of other emission control devices (e.g., oxidationcatalyst, first SCR region, etc.). Other techniques for increasing ordecreasing the temperature of the second SCR region may be employed,such as use of one or more heat exchangers.

Referring now to FIG. 14, the method 1100 is the fourth emission controlstrategy, which may be carried out if the storage capacity of both thefirst and second SCR regions are less than respective storage capacitythresholds (e.g., not desired storage capacities), such as when at leastone of the SCR regions is degraded. Here, a temperature of the first SCRregion may be adjusted to be within a first desired range, and atemperature of the second SCR region may be adjusted to be within asecond desired range.

Specifically, at 1102, it is determined if a temperature of the firstSCR region is above an upper temperature threshold T_(1H). If the answeris no at 1102, the first SCR region is not yet warmed up, such as atpoint A of FIG. 9. The method 1100 may proceed to 1104 where exhaustheat temperature is increased to thereby warm up the first SCR region.Some ways that exhaust heat temperature can be controlled may includeadjusting an air-fuel ratio of combustion chamber contents, injectiontiming, throttling, etc.

Also, the method 1100 may include stopping the injecting of reductant at1104. It may be desirable to stop the injecting since the storagecapacity of both of the SCR regions is less than a desired amount, and alikelihood of reductant slipping to the atmosphere may be high.

If the answer is yes at 1102, the temperature of the first SCR regionmay be too high, such as at point B of FIG. 9. Thus, the method 1100proceeds to 1106, where the exhaust heat temperature may be reduced inorder to cool the first SCR region.

At 1108, the method 1100 may include determining if a temperature of thesecond SCR region is greater than an upper threshold T_(2H). If theanswer is no at 1108, this indicates the second SCR region may be toocold (e.g., point A of FIG. 9), so the method 1100 proceeds to 1110,where exhaust heat may be increased to warm up the second SCR region. Ifthe answer is yes at 1108, the second SCR region may be too hot (e.g.,point B of FIG. 9), so the method 1100 may proceed to 1112 where theexhaust temperature may be reduced in efforts to cool the second SCRregion.

From the discussions of the emission control strategies presented inFIGS. 11-14, it may be appreciated that, under some conditions, anemission control system may be operated with a first amount of storedreductant at the first SCR region greater than a second amount of storedreductant at the second SCR region, such as during the third emissioncontrol strategy, during an engine cold start, or when a temperature ofthe second SCR region is below a lower threshold (e.g., too cold) orabove an upper threshold (e.g., too hot), as just some examples.

In some cases, there may be more stored reductant at the first SCRregion than the second SCR region by happenstance (e.g., as a result ofnormal engine operation). In other examples, operating parameters may beadjusted to achieve the first amount of stored reductant greater thanthe second amount of stored reductant. Said operating parameters mayinclude one or more of a NOx flow, a temperature of the first SCRregion, a temperature of the second SCR region, and an amount ofreductant injected as just some examples. Thus, in order to achieve agreater amount of first stored reductant than second stored reductant,NOx flow to the first SCR region may be reduced, temperature of thefirst SCR region may be maintained in a predetermined range, and/or anamount of reductant injected may be increased.

On the other hand, an emission control system may be operated with thesecond amount of stored reductant greater than the first amount ofstored reductant, such as when a temperature of the first SCR region isabove an upper threshold (e.g., too hot), or when the first SCR regionis disabled, or when performing a SCR filter (or diesel particulatefilter) regeneration, as some examples. In some cases, there may be morestored reductant at the second SCR region than at the first SCR regionsimply as a result of engine operation, or happenstance. This may occurwhen vehicle acceleration and/or engine speed is above a predeterminedthreshold. For example, during hard acceleration and high speed driving,an efficiency of the first SCR region may be lower than 100% due to hightemperature and high space velocity of the first SCR region. In thiscase, the second SCR region may be in a temperature window sufficientfor unconverted NOx received from the first SCR region to be consumed bystored NH₃ at the second SCR region.

In other examples, operating parameters may be adjusted to cause thefirst amount of stored reductant to be less than the second amount ofstored reductant. As discussed, said operating parameters may includeone or more of a NOx flow, a temperature of the first SCR region, atemperature of the second SCR region, and an amount of reductantinjected. In order to achieve a lesser amount of first stored reductantthan second stored reductant, NOx flow to the first SCR region may bereduced compared to NOx flow at the second SCR region, temperature ofthe first SCR region may be increased above an upper threshold, and/ortemperature of the second SCR region may be maintained in apredetermined range. Furthermore, an amount of reductant injected may beincreased or decreased based on other conditions, in order to achieve agreater amount of reductant stored at the second SCR region compared tothe first SCR region.

FIG. 15 shows, as a schematic graph, an amount of reductant at a midbedlocation (e.g., between the first SCR region and the second SCR region)and an amount of reductant at a tailpipe location (e.g., downstream ofthe second SCR region) during operation of an engine and an emissioncontrol system configured to use the emission control strategiesdescribed herein. It can be appreciated from this graph that the use ofmore than one SCR region in combination with the control strategiesdescribed herein substantially reduces reductant slip.

Specifically, FIG. 15 represents, prophetically, a “typical” fieldcondition. The trace shows the ammonia release over a FTP 75 cycle. Inthis example, the first SCR region and the second SCR region havemoderate-to-high pre-adsorbed levels of ammonia. The 0-505 second regionis a bag 1 cold start—with some over-injection of NH3 to boostconversion with a small amount of slip. The larger ammonia spike is frombag 3 of the FTP—hot-start and the release from the first SCR istemperature driven and the second is due to pre-adsorbed ammonialimiting some of the adsorption. In contrast, in the condition of nopre-adsorbed ammonia, with the over-injection strategy described herein,there is a similar trace as the solid line, but the dashed line does nothave any spikes over 20 ppm since the second SCR region is adjusted tohave substantially no ammonia stored on it and can completely adsorb thespikes.

Turning now to the issue of diagnosing emission control systemcomponents in an emission control system having more than one SCR regionin series, FIG. 16 illustrates a method 1200. The method 1200 includesreading operating conditions at 1202. Operating conditions may includeengine operating conditions and/or a current emission control strategy,as some examples. At 1202, it is determined that there is a problem withthe emission control system, and that the problem is not a problem ofthe reductant injector nor is it a problem of any of one or more sensorsthat may be sensing a condition of emission control system components.That is, emission control components other than the first SCR region andthe second SCR region may have already been ruled out (or diagnosed) ascandidate contributors to the emission control system problem.

As will be discussed in detail, diagnostics may be performed when one ofthe SCR regions is not converting NOx, is converting NOx at anunacceptably low rate (e.g., less than 50%), or at a known rate. Thus,diagnostics may be performed when a temperature of the first and/orsecond SCR region is too low or too high so as to be converting NOx. Forexample, if the first SCR region is outside of a NOx converting range,the second SCR region can be diagnosed. On the other hand, if the secondSCR region is outside of a NOx converting range, the first SCR regioncan be diagnosed. An operating range may correspond to a temperaturerange and/or a storage capacity of an SCR region.

Although not shown, the method 1200 may include adjusting operatingparameters to achieve a desired temperature of the first SCR regionand/or a desired temperature of the second SCR region, where theoperating parameters may include one or more of an engine speed, anengine load, an air-fuel ratio, and an amount of reductant injected.That is, the temperature of the first and/or second SCR region may bepurposefully increased or decreased in order to place the system in astate conducive to performing SCR system diagnostics.

At 1204, the method 1200 includes determining if a temperature of thefirst SCR region is greater than an upper temperature threshold T_(1H).If the answer is yes at 1204, this may indicate the first SCR region isdisabled (e.g., unable to convert NOx). Thus, the method 1200 mayproceed to 1206, and determine if a temperature of the second SCR regionis above a lower threshold T_(2L). The lower threshold T_(2L) may be atemperature below which the second SCR region is unable to storereductant and/or unable to convert NOx. Thus, if the answer is yes at1206 (e.g., the second SCR region is converting NOx), the methodproceeds to 1208.

At 1208, the method 1200 includes evaluating the performance of thesecond SCR region. Since the first SCR region may be functionallydisabled under these conditions, diagnostics of the second SCR regioncan be carried out under an assumption that a majority of the engineoutput NOx is flowing to the second SCR region without being convertedat the first SCR region. This may occur, for example, during an SCRfilter regeneration, when the first SCR region is being subjected tohigh temperatures. That is, during an SCR regeneration cycle, thetemperature of the first SCR region may rise, and thus the reductantstorage capacity decreases.

At 1210, the method 1200 includes reading a sensor signal from a NOxsensor downstream of a second SCR region, for example. At 1212, themethod 1200 includes determining a storage capacity of the second SCRregion. The method 1200 may also include determining a NOx conversionefficiency of the second SCR region at 1214. Various calculations and/orcomparisons may be made to evaluate the performance of the second SCRregion.

At 1216, if it is determined that the second SCR region's performance isless than an expected performance (e.g., the performance is not as goodas expected), the method 1200 includes setting a diagnostic of thesecond SCR region at 1218. As one example, the setting of a diagnosticof the second SCR region may be carried out responsive to determiningthat a storage capacity of the second SCR region is less than anexpected storage capacity of the second SCR region. That is, the method1200 includes setting a diagnostic of the second SCR region based on asignal indicating a degradation of the second SCR region.

However, if the second SCR region's performance is at least equal to theexpected performance (e.g., the performance is as good as expected), theperformance of the second SCR region may not be the source of theemission control system problem. Thus, the method 1200 includes settinga tentative first SCR region diagnostic at 1220. That is, the method1200 may include setting a diagnostic of the first SCR region based on asignal indicating there is not degradation of the second SCR region.

If the answer is no at 1206, such that the temperature of the first SCRregion is above an upper threshold T_(1H) (e.g., first SCR region is toohot), and the temperature of the second SCR region is below the lowerthreshold T_(2L) (e.g., second SCR region is too cold), the method 1200may end. That is, emission control system diagnostics may not beperformed, in this example. However, it may be appreciated that, in asystem with provisions for doing so (e.g., additional sensors), emissioncontrol system diagnostics may be carried out under such conditions.

If the answer is no at 1204, such that the temperature of the first SCRregion is not above the upper threshold T_(1H) (e.g., the first SCRregion is converting NOx), the method 1200 proceeds to 1222. Here, it isdetermined if the second SCR region is below a lower threshold T_(2L).If the answer is yes at 1222, such that the first SCR region isconverting NOx but the second SCR region is not converting NOx,diagnostics of the first SCR region can be carried out under anassumption that the second SCR region is not substantially contributingto NOx conversion. These conditions may occur, for example, during atleast a portion of an engine cold start, when the first, but not thesecond, SCR region is sufficiently warmed up.

At 1224, the method 1200 includes evaluating a performance of the firstSCR region. This may include reading a sensor signal at 1226,determining the first SCR region's storage capacity at 1228, and/ordetermining NOx conversion efficiency by the first SCR region at 1230.

At 1232, the first SCR region's performance is compared against anexpected performance. As one example, the performance of the first SCRregion may be a storage capacity, based on the temperature of the firstSCR region, and this may be compared against an expected storagecapacity. In other examples, an amount of NOx downstream of the firstSCR region may be compared to an expected amount of NOx, and/or a NOxconversion efficiency of the first SCR region may be compared against anexpected NOx conversion efficiency.

In any case, if the first SCR region's performance is less than theexpected performance (e.g., not converting NOx at an expected rate), themethod 1200 includes setting a diagnostic of the first SCR region at1234. That is, the method 1200 includes setting a diagnostic of thefirst SCR region based on a signal indicating a degradation of the firstSCR region.

If the answer is no at 1232, the method 1200 includes setting atentative second SCR region diagnostic at 1236. That is, the method 1200may include setting a diagnostic of the second SCR region based on asignal indicating there is not degradation of the first SCR region.

If the answer is no at 1222, such that both of the first SCR region andthe second SCR region are converting NOx at a substantially high rate,the emission control diagnostics may end.

As suggested above, additional, or supplementary, emission controldiagnostic routines may be provided for performing diagnostics of thefirst and/or second SCR region when both of the regions are operable andconverting NOx at a substantially high rate. Such a routine may rely onone or more sensors (e.g., NOx sensors) downstream of the first and/orsecond SCR regions. In this way, even when both SCR regions are withinoperable ranges, degradation of one or more of the SCR regions can bedetermined by comparing an amount of NOx conversion by each region(detected by a NOx sensor) to an expected NOx conversion efficiency. Anexpected conversion efficiency may be based on one or more of engineload, engine speed, storage capacity, amount of stored reductant,temperature, etc.

As discussed above, an oxidation catalyst may be positioned downstreamof the first SCR region and upstream of the second SCR region, and thusmay oxidize at least a portion of reductant slipped from the first SCRregion. In such a case, the method 1200 may include provisions fordetermining if the oxidation catalyst is performing as expected. In somecases, the method 1200 may include setting an oxidation catalystdiagnostic based on an oxidation catalyst performance when thetemperature of the first SCR region is above the upper threshold T_(1H)at 1204 and the temperature of the second SCR region is above the lowerthreshold T_(2L) at 1206. In such a case, substantially all of theengine output NOx may be received at the oxidation catalyst. Thus, anelectronic controller may set an expected performance of the oxidationcatalyst (e.g., how much NOx will be created) and an expectedperformance of the second SCR region (e.g., how much NOx will beconverted), and compare an amount of tailpipe NOx to an expected amountof NOx. In yet other examples, an oxidation catalyst may be diagnosed ifthe answer to 1216 and/or 1232 is no.

The method 1200 is exemplary. As such, the determination at 1204 may bereplaced with a determination of whether the temperature of the firstSCR region is below a lower threshold that functionally disables thefirst SCR region, or it may be replaced with any other determinationthat the first SCR region is insubstantially contributing to NOxconversion. Similarly, the determination at 1222 may be replaced with adetermination of whether the temperature of the second SCR is greaterthan an upper threshold that functionally disables the second SCRregion, or it may be replaced with any other determination that thesecond SCR region is insubstantially contributing to NOx conversion.Further, the determination at 1206 may be replaced with a determinationthat the second SCR region is within an operating range, or that it issubstantially contributing to NOx conversion.

Note that the example control and estimation routines included hereincan be used with various engine and/or vehicle system configurations.The specific routines described herein may represent one or more of anynumber of processing strategies such as event-driven, interrupt-driven,multi-tasking, multi-threading, and the like. As such, various acts,operations, or functions illustrated may be performed in the sequenceillustrated, in parallel, or in some cases omitted. Likewise, the orderof processing is not necessarily required to achieve the features andadvantages of the example embodiments described herein, but is providedfor ease of illustration and description. One or more of the illustratedacts or functions may be repeatedly performed depending on theparticular strategy being used. Further, the described acts maygraphically represent code to be programmed into the computer readablestorage medium in the engine control system.

It will be appreciated that the configurations and routines disclosedherein are exemplary in nature, and that these specific embodiments arenot to be considered in a limiting sense, because numerous variationsare possible. For example, the above technology can be applied to V-6,I-4, I-6, V-12, opposed 4, and other engine types. Further, whilevarious portions are described with respect to controlling/adjustingreductant injection to adjust ammonia delivered to the various SCRregions, reductant generation in an upstream catalyst, such as anupstream lean NOx trap, e.g., via adjustment of a rich exhaust gasair-fuel ratio, may also be used in place of, or in addition to, thereductant injection. Thus, for each instance of adjusting reductantinjection in a particular way, adjustment of air-fuel ratio in anupstream lean NOx trap may also be used in place thereof. As such, thesubject matter of the present disclosure includes all novel andnonobvious combinations and subcombinations of the various systems andconfigurations, and other features, functions, and/or propertiesdisclosed herein.

The following claims particularly point out certain combinations andsubcombinations regarded as novel and nonobvious. These claims may referto “an” element or “a first” element or the equivalent thereof. Suchclaims should be understood to include incorporation of one or more suchelements, neither requiring nor excluding two or more such elements.Other combinations and subcombinations of the disclosed features,functions, elements, and/or properties may be claimed through amendmentof the present claims or through presentation of new claims in this or arelated application. Such claims, whether broader, narrower, equal, ordifferent in scope to the original claims, also are regarded as includedwithin the subject matter of the present disclosure.

The invention claimed is:
 1. A method for controlling an emissioncontrol system having a first SCR region upstream of a second SCRregion, the method comprising: operating in a first mode if a storagecapacity of the first SCR region is greater than a first threshold and astorage capacity of the second SCR region is not greater than a secondthreshold, the first mode including adjusting an amount of reductantdelivered upstream of the first SCR region based on a condition of thefirst SCR region; and operating in a second mode if the storage capacityof the first SCR region is not greater than the first threshold and thestorage capacity of the second SCR region is greater than the secondthreshold, the second mode including adjusting the amount of reductantdelivered upstream of the first SCR region based on a condition of thesecond SCR region.
 2. The method of claim 1, wherein the condition ofthe first SCR region is an amount of stored reductant at the first SCRregion, and the condition of the second SCR region is an amount ofstored reductant at the second SCR region.
 3. The method of claim 1,further comprising operating in the first mode when a storage capacityof the first SCR region is greater than a storage capacity of the secondSCR region, and operating in the second mode when the storage capacityof the first SCR region is smaller than the storage capacity of thesecond SCR region, where the first and second SCR regions are positionedin separate emission control devices.
 4. The method of claim 3, wherein,in the first mode, the adjusting includes reducing an amount ofreductant injected when an amount of stored reductant in the first SCRregion is above a first threshold amount, and wherein, in the secondmode, the adjusting includes reducing the amount of reductant injectedwhen an amount of stored reductant in the second SCR region is above asecond threshold amount.
 5. The method of claim 2, further comprisingincreasing engine output NOx based on one or more of the amount ofstored reductant at the first SCR region being above a first thresholdamount and the amount of stored reductant at the second SCR region beingabove a second threshold amount.
 6. The method of claim 1, wherein thesecond mode includes the first SCR region being below a lowertemperature threshold or above an upper temperature threshold.
 7. Themethod of claim 1, where, in the first mode, the adjusting of the amountof reductant delivered is further based on the condition of the secondSCR region.
 8. The method of claim 1, where the second mode furtherincludes: disabling the first SCR region to increase NOx flow to thesecond SCR region based on an amount of reductant stored at the secondSCR region being greater than a threshold amount.
 9. The method of claim8, wherein the disabling includes increasing a temperature of the firstSCR region and/or decreasing an amount of stored reductant at the firstSCR region.
 10. The method of claim 1, further comprising operating in athird mode if the storage capacity of the first SCR region is greaterthan the first threshold and the storage capacity of the second SCRregion is greater than the second threshold, the third mode including:when an amount of stored reductant at the second SCR region is greaterthan a storage threshold, increasing a temperature of the first SCRregion and reducing an amount of reductant delivered upstream of thefirst SCR region; and when the amount of stored reductant at the secondSCR region is not greater than the storage threshold, adjusting theamount of reductant delivered upstream of the first SCR region based onan amount of stored reductant at the first SCR region.
 11. The method ofclaim 1, further comprising operating in a fourth mode if the storagecapacity of the first SCR region is not greater than the first thresholdand the storage capacity of the second SCR region is not greater thanthe second threshold, the fourth mode including: adjusting exhausttemperature based on a temperature of the first SCR region and atemperature of the second SCR region.
 12. An emission control system foran internal combustion engine comprising: a reductant injector; a firstSCR region positioned downstream of the reductant injector; a second SCRregion positioned downstream of the first SCR region; and an electroniccontroller for: in a first mode, adjusting reductant injected based onan amount of stored reductant at the first SCR region, and adjustingexhaust temperature based on a temperature of the second SCR region, andin a second mode, adjusting reductant injected based on an amount ofstored reductant at the second SCR region, and adjusting exhausttemperature based on a temperature of the first SCR region.
 13. Thesystem of claim 12, wherein the first SCR region has a first washcoatdensity greater than a second washcoat density of the second SCR region.14. The system of claim 12, where the first SCR region has a firstsurface area-volume ratio that is greater than a second surfacearea-volume ratio of the second SCR region.
 15. The system of claim 12,wherein the first SCR region is a first zone of an SCR catalyst and thesecond SCR region is a second zone of the SCR catalyst, the first zoneand the second zone being positioned adjacent to one another.
 16. Thesystem of claim 12, further comprising an oxidation catalyst positionedbetween the first SCR region and the second SCR region such that theoxidation catalyst receives slipped reductant from the first SCR regionand oxidizes at least a portion of the slipped reductant.
 17. The systemof claim 12 wherein, in the first mode, adjusting exhaust temperaturebased on the temperature of the second SCR region comprises increasingexhaust temperature when the temperature of the second SCR region isbelow a second temperature threshold and decreasing exhaust temperaturewhen the temperature of the second SCR region is above the secondtemperature threshold, and wherein, in the second mode, adjustingexhaust temperature based on the temperature of the first SCR regioncomprises increasing exhaust temperature when the temperature of thefirst SCR region is below a first temperature threshold and decreasingexhaust temperature when the temperature of the first SCR region isabove the first temperature threshold.
 18. A method for an emissioncontrol system having a first SCR region upstream of a second SCRregion, the method comprising: only when an amount of stored reductantat the second SCR region is greater than a threshold, increasing atemperature of the first SCR region and reducing an amount of reductantinjected upstream of the first SCR region; and only when the amount ofstored reductant at the second SCR region is not greater than thethreshold, adjusting the amount of reductant injected upstream of thefirst SCR region based on an amount of stored reductant at the first SCRregion.
 19. The method of claim 18 further comprising, when the amountof stored reductant at the second SCR region is not greater than thethreshold, adjusting the amount of reductant injected based on theamount of stored reductant at the second SCR region.
 20. The method ofclaim 18, where the adjusting includes reducing the amount of reductantinjected upstream of the first SCR region before the increasing of thetemperature of the first SCR region.
 21. The method of claim 18 furthercomprising, when the amount of stored reductant at the first SCR regionis greater than a threshold, reducing the amount of reductant injectedupstream of the first SCR region and increasing NOx flow to the firstSCR region.